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(Mechanical effects of light on anisotropic micron-sized particles and their wetting dynamics at the water-air interface

Abstract : We report experiments on ellipsoidal micrometre-sized polystyrene particles. The particle aspect ratio (k) varies between about 0.2 and 8. These particles are manipulated in water by means of a moderately focused laser beam. We observe the levitation and the dynamical state of each particle in the laser beam, in bulk water or in contact to an interface (water-glass, water-air, water-oil). In the first part, we show that moderate-k particles are radially trapped with their long axis lying parallel to the beam. Conversely, elongated (k>3) or flattened (k<0.3) ellipsoids never come to rest, and permanently “dance” around the beam, through coupled translation-rotation motions. The dynamics are periodic or irregular (akin to chaos) depending on the particle type and beam characteristics. We propose a 2d model that indeed predicts the bifurcation between static and oscillating states. In the second part, we apply optical levitation to study the transition from total to partial wetting of the particles at the water-air interface. We show that the dynamics of the transition is about independent of particle shape, and mainly governed by the pinning-depinning mechanism of the contact line.
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Submitted on : Thursday, October 3, 2013 - 1:02:03 AM
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Besira Mihiretie. (Mechanical effects of light on anisotropic micron-sized particles and their wetting dynamics at the water-air interface. Other. Université Sciences et Technologies - Bordeaux I, 2013. English. ⟨NNT : 2013BOR14813⟩. ⟨tel-00869331⟩

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